1. Field of Invention
The present invention relates to an AC/DC converter, and more particularly to a single-stage isolated high power factor AC/DC converter with a leakage inductor energy recovery function.
2. Related Art
A conventional AC/DC converter generally is the following structure: a boost converter, a buck converter, or a buck-boost converter is used as a first-stage power factor correction (PFC) circuit, and a forward converter or a flyback converter is used as a second-stage driving circuit, so as to achieve electrical isolation and convert an output voltage to a set level, thus providing a stable voltage for driving a load. For the conventional two-stage architecture, two converter circuits with independent control are required, so the circuit cost is high; meanwhile, two power conversion stages also lead to lower circuit efficiency.
FIG. 1 shows a two-stage isolated high power factor AC/DC converter that includes a buck-boost PFC circuit and a flyback converter in the prior art. The two-stage isolated high power factor AC/DC converter is formed by a flyback converter 140 and a buck-boost PFC circuit 130, and includes a filter circuit 110 for filtering an AC power supply Vac. The filter circuit 110 is formed by a filter inductor Lf and a filter capacitor Cf. The filtered power supply is rectified by a rectification circuit 120 formed by diodes Dr1, Dr2, Dr3, and Dr4. The buck-boost PFC circuit 130 is formed by an inductor Lb, a capacitor Cdc, a diode D, and a switch S1. The flyback converter 140 draws energy from the capacitor Cdc through high-frequency switching of a switch S2, and delivers the energy to a secondary side via a transformer T, thus changing the voltage level and achieving the electrical isolation. The buck-boost PFC circuit controls an input current through high-frequency switching of a switch S1, thus achieving the PFC effect. An output rectifier Do and an output capacitor Co are used for filtering.
FIG. 2 shows a two-stage isolated high power factor AC/DC converter that includes a buck-boost PFC circuit and a forward converter in the prior art. Like numbers used in this figure refer to like parts in FIG. 1.
A forward converter 141 draws an energy from a capacitor Cdc through high-frequency switching of a switch S2 and delivers the energy to a secondary side via a transformer, thus changing the voltage level and achieving the electrical isolation. The buck-boost PFC circuit controls an input current through high-frequency switching of a switch S1, thus achieving the PFC effect.
Current PFC circuits operate at a frequency ranging from tens to hundreds of kHz, allow for considerable ranges of variation in the input power supply and load, can inhibit harmonic distortion to almost none, and have an unity power factor. The basic circuit architecture of a DC/DC converter may be classified into six basic types according to relative positions of an energy storage inductor and an active switch, namely, a buck converter, a boost converter, a buck-boost converter, a Cúk converter, a SEPIC converter, and a Zeta converter. Boost and buck-boost circuit architectures are suitable for implementing the PFC. No matter the energy storage inductor operates in a continuous current mode (CCM) or a discontinuous current mode (DCM), the high power factor correction can be achieved. For the same output power, the inductor operating in the DCM has a peak current greater than that in the CCM. The higher the power is, the greater the peak current is, and the switching loss of the circuit also increases accordingly. Therefore, the CCM is suitable for high power output. However, when the inductor operates in the CCM, a control circuit must detect the relations among the input voltage, the inductor current, and the output voltage in real time, so the circuit is complex. In addition, the switching frequency and duty ratio of the switch must be constantly changed in every input voltage cycle. If the PFC circuit and the second-stage converter need to be integrated into the single-stage architecture, the switch elements of the PFC circuit and the second-stage converter must have the same switching frequency and duty ratio. Therefore, when the PFC circuit operates in the CCM, the PFC circuit is not suitable for being integrated with the second-stage converter. In contrast, for the buck-boost PFC converter, if the switching frequency and duty ratio of the switch element thereof are kept constant in every input power supply cycle, the PFC function can be easily achieved when the inductor operates in the DCM.
However, when the flyback converter is used, since a leakage inductor of the transformer is large due to the operating principles and design of the circuit of the flyback converter, much energy is stored in the leakage inductor of the transformer. In FIG. 1, when the active switch S2 of the flyback converter is cut off to deliver the energy to the secondary side of the transformer, there is no way for discharging the energy stored in the leakage inductor at the primary side of the transformer. At this time, a large surge is generated, causing a significant circuit loss, and thus reducing the circuit efficiency. Therefore, in recent years, many researchers are devoted to studying the discharge of the energy stored in the leakage inductor at the primary side of the transformer, and have proposed many methods, for example, a technology such as “active clamp”. The active clamp technology is to capture the leakage energy stored in the leakage inductor at the primary side of the transformer by using a clamping capacitor, and then recycle the energy to a load and back to the input end via a system, thus generating a nearly loss-free buffer. As such, the problem caused by the leakage inductor of the flyback converter can be solved and the circuit efficiency can be greatly improved. However, the active clamp technology requires adding at least one active switch and one capacitor, which increases the circuit cost and makes the control complex.
Moreover, since the designs of FIG. 1 and FIG. 2 need two control circuits and two active switches, the circuit cost is further increased.